Mixed lithography is a term broadly applied to lithographic processes that use two or more different exposure sources to create a composite image in a single layer of resist. Once the lithographic patterning is complete, the composite image is developed and the pattern is transferred into an underlying layer. This process can be used to exploit advantages of different lithography techniques without employing two separate pattern transfer steps. A common example of mixed lithography involves combining deep ultra violet (DUV) photolithography and electron beam lithography (EBL) in a single layer of resist.
Negative-tone chemically amplified resists are commonly used in mixed lithography processes combining DUV photolithography and electron beam lithography. When a negative tone resist is employed in lithography, portions of the resist that are exposed to a light beam remain on an underlayer, while unexposed portions of the resist are removed during resist development. A collective set of remaining portions of the resist is subsequently used as an etch mask for pattern transfer into the underlayer.
A typical process flow for mixed lithography using a negative tone chemically amplified resist is shown below:
(a) Application of a resist and performing a post application bake (PAB)
(b) Exposure of the resist with an electron beam and a first post-exposure bake (PEB)
(c) Exposure of the resist with a DUV beam and a second post-exposure bake (PEB)
(d) A single RIE process to transfer the pattern in the resist into one or more underlying layers on a substrate.
FIGS. 1A-1F show sequential vertical cross-sectional views of an exemplary prior art structure showing processing steps of conventional mixed lithography employing a negative-tone chemically amplified resist. The negative tone chemically amplified resist comprises at least one polymeric compound that is sensitive to both electron beam exposure and ultraviolet exposure. An example of the negative tone chemically amplified resist is commercially available NEB-22A5™ resist from Sumitomo Chemical Company.
Referring to FIG. 1A, a negative tone chemically amplified resist is applied to an underlying layer 10 to form a negative tone chemically amplified resist layer 20. The negative tone chemically amplified resist layer 20 consists of unexposed negative tone resist region 21, which is contiguous throughout the negative tone chemically amplified resist layer 20 at this point. Typically, the thickness of the negative tone chemically amplified resist layer 20 may be about 100 nm or greater. A post-application bake is performed to stabilize the negative tone chemically amplified resist layer 20 at a temperature from about 70° C. to about 150° C. for a time period from about 30 seconds to about 120 seconds. The process parameters of the post-application bake may be optimized for performance.
Referring to FIG. 1B, portions of the negative tone chemically amplified resist layer 20 are exposed with an electron beam to form a pattern of electron beam exposed negative tone resist regions 22A, i.e., a pattern of exposed portions of the negative tone chemically amplified resist layer 20, within a template of unexposed negative tone resist region 21, which excludes the exposed portions of the negative tone chemically amplified resist layer 20. Due to smaller wavelength of the electron beam, the resolution of the electron beam exposed pattern is higher than the resolution that is achievable with ultraviolet radiation. Electron beam radiation induced chemical changes occur in the electron beam exposed negative tone resist regions 22A. The electron beam radiation induced changes may be cross-linking of copolymers, chain scission, ring opening of an aryl moiety, or a combination thereof. A first post-exposure bake is performed to diffuse photoacid in the negative tone chemically amplified resist layer 20 and to facilitate the radiation induced changes. For example, the first post-exposure bake may be performed at a temperature from about 70° C. to about 150° C. for a time period from about 30 seconds to about 120 seconds.
Referring to FIG. 1C, portions of the negative tone chemically amplified resist layer 20 are exposed with an ultraviolet (UV) radiation in a conventional ultraviolet lithography to form a pattern of template of unexposed negative tone resist region 21. UV radiation induced chemical changes occur in the UV exposed negative tone resist regions 22B. The UV radiation induced changes may be cross-linking of copolymers, chain scission, ring opening of an aryl moiety, or a combination thereof, as in the electron beam exposed negative tone resist regions 22B. A first post-exposure bake is performed to diffuse photoacid in the negative tone chemically amplified resist layer 20 and to facilitate the radiation induced changes. For example, the first post-exposure bake may be performed at a temperature from about 70° C. to about 100° C. for a time period from about 30 seconds to about 120 seconds.
Referring to FIG. 1D, the negative tone chemically amplified resist layer 20 is developed to remove the template of unexposed negative tone resist region 21 selective to exposed negative tone resist regions 22, which comprise the electron beam exposed negative tone resist regions 22A and the UV exposed negative tone resist regions 22B. After the development, the negative tone chemically amplified resist layer 20 consists of exposed negative tone resist regions 22.
Referring to FIG. 1E, the negative tone chemically amplified resist layer 20 is employed as an etch mask during a reactive ion etch to form trenches in the underlying layer 10 corresponding to the pattern in the negative tone chemically amplified resist layer 20.
Referring to FIG. 1F, the negative tone chemically amplified resist layer 20 is removed. The underlying layer 10 has a combined pattern of the electron beam lithography and the conventional ultraviolet lithography.
Two primary problems exist with the negative-tone chemically amplified resist-based mixed lithography scheme. (1) Only a single exposure process can be optimized at any one time. For example, good processing parameters for the electron beam exposure preclude the patterning of high resolution features using optical lithography and visa versa. (2) High resolution features at dense pitches experience pattern collapse due to mechanical failure of the resist structures during the develop process. This is driven by the density and aspect ratio of the developed resist features and the capillary force created during the post develop rinsing.
FIG. 2 shows an example of a failed pattern in which portions of a developed pattern of a negative tone chemically amplified resist layer collapsed. In this example, 30 nm wide lines having a pitch of 100 nm consistently fail to yield for the negative tone chemically amplified resist layer having a thickness of 100 nm or greater. While thinning down the negative tone chemically amplified resist layer can mitigate this problem, the performance of chemically amplified resists degrades as the film thickness decreases.
In view of the above, there exists a need for methods of mixed lithography with which patterns of electron beam lithography and ultraviolet lithography may be printed without loss of resolution, and structures for the same.
Further, there exists a need for methods of mixed lithography for preventing or circumventing pattern collapses in the resist to enable printing of high resolution features at dense pitches, and structure for the same.